Process for producing calcium sulphate-zinc sulphide pigments



Patented Sept. 3, 1940 UNITED STATES PROCESS FOR PRODUCING CALCIQUMS-UL- v rnsrazmc SULPHIDE rig vmn'r Roy William Sullivan, Cragmere, llelas slgnor i; I

to E. I. du Pont de Nemours & Company, W11- mington, Del., a corporationof Delaware No Drawing. Application October 21, 1937 Serial No.170,221'

4Claims. (01.13pm), f.

This invention relates to the manufacture of composite calciumsulphate-zinc sulphide pigments, and more particularly to a process forobtaining said pigments by precipitation from 5 solutions containingcalcium and sulphide ions.

More specifically, the invention has reference to the provision of aneconomical and simple method for producing calcium base lithoponepigments from calcium chloride solutions by reacting said solutions witha soluble sulphide and zinc salt, and at the same time inherentlyavoiding the redissolving of the calcium sulphate formed orprecipitation of calcium sulphide.

The difiiculties heretofore encountered in the production of calciumsulphate-zinc sulphide pigments have been due mainly to theinsolubility' of calcium sulphide in aqueous solvents. In this respect,such processes are essentially difierent from common processes forproducing barium lithopone pigments, where the corresponding bariumsulphide is readily soluble in water and where solutions of zincsulphate can thus be allowed to react with solutions of barium sulphide.Because of the diverse properties of barium sulphide, as compared withcalcium sulphide, processes for the production of barium lithopone arenot interchangeable with or adapted to the production of calciumlithopone.

Thus, barium sulphide solutions of high concen- I tration, e. g.,several hundred grams BaS/ 1, are readily prepared by leaching bariumblack ash, a product obtained by thermal reduction of barytes. On theother hand, calcium sulphide obtained in similar manner by furnacingcalcium sulphate is practically insoluble .in waterand impure solutionscontaining only about 3 g. 0218/ 1 are attainable. Furthermore, bariumsulphate is practically insoluble and does not precipitate in thehydrated form. When precipitated in the conventional manner as aconstituent of barium lithopone there is no appreciable tendency forparticle size growth even on long continued standing in aqueous media.However, calcium sulphate precipitates in the form of the dihydrate andis over 12,000 times as soluble as barium sulphate. Additionally, theparticles of calcium sulphate tend to grow rapidly to" such.

large proportions that they become unsuited for pigments.

. As is well-known, the value of pigmentsand extenders is dependent uponparticle size which influences to a great extent the tinting strength,hiding power and oil absorption properties of the pigment. Particle sizeof both the prime '55 pigment and the extender portion of a compositepigment is also an important factor in the obtainment of good filmfineness, gloss, and other paint film properties. Similarly, good coloris an important requisite for a marketable white qo pigment.

the well-known. ma

I have found t-ha' tall of 'these desirable-:attributes inherentlyresult from a practiceyor" my invention. a

' I havealso foun that calcium fsulphide ls-not precipitated fromsolutionsjof a calcium salt and -5 ammonium sulphide; 'afriliibhalfithe'CaSOlformed W111 maybe subject-to redissolution-to any jappreciableextent; Accordingly, the preferred embodiment of invention comprisesreacting a solution" containing avafilable' zinc; and sull0 phateion's"with" an aqueous solution containing ammonium sulphide and asoluble calcium salt, as a result of which a co -precipitatedcalcium'sulphate and zinc sulphide pigment isobtained. I have round"further," atftlie precipitated; cal- :15 cium sulphate'of es's isvery'fine inparticle size, containsno r gypsum crystals, and is eminentlyusefii extender; for the zinc sulphide. Seemingly, restrai ingfinfiuenceis exertedupon" the parti'c lefsi'ze vgolf the calciu'm 20 su p at e i db ilnbf'ba I i m Q Su ph ions present in the"fzinc.jsolution; h

In the preferred embodiment of i my invention, I. add to asolution of"sulphide any desiredamount iumphloride, calcium 5 nitrate e'rgpmerfls ucalcium, salts obtained from an'acidjhaving radicals "wm nrdrm solublesalts .With'jzinc', ,ahd'jeiflrildy this mixture "to "fit" lcium'baselithopone .To the ammoniuml sulphide calcium 5 30 salt Samson, I add a'sna p containing available zinc and sulphate jion s, a a result ofwhichinteraction between tn 'z'i and, sulphide ions and thecalciuin and sulpI e 1ons..00gur, A precipitation "and ,alkalimty "adius'tment, the"corn- 35 posite 'sulphide,; and, hydrated. calcium J sulwei djan' rwshed andtis velop pigment properties ing. or. disintegration' ih thenheat treated, 'to and comminutedby An ,additional advant ge' games ythe-jinvention is the employment pf ammonium sulphide s the zinc.sulphide Lpre ipitam permitun recovery'erjthe' ammonium chloride forreuse in the process, aha -corise quent ly afmeth od is afiord- 5 ed in,which cyclic perati onresults.l,

My novel process, s illustrated by means? of the following chemicalequations:

- (b) 2 1 m o'1"+ca on zeoaonmunion ZN-I QHt-H T? N w j Asuitable;modificationiofithe foregoing process involves reaction-of thezinc solution directly WlthF the solution :containing both calcium; andsulphide-ions. For instance, crude calcium sulphide maybe leached underpressure. at: elevated temperatures'withammonium. chloride solution andthe clarified.-solutionccontaining calcium Ich1o-;;go

ride, ammonium. and sulphidelions may then be reacted-vwitheavzinc:solutionccontaining the sulphate 'ionin necessary amount. Thismodification also involves a cyclic operation and is readily 5understandable from the following equations:

givengnonerofxwhieht are toibetconsidered eas in z-anyiwise limitingthe'scope oi. the invention:

.EExampIaI Q .1350 .0: zinc .asiilphate solution of 39 Baum equivalent'-.to i528.6, parts' by weight .of zinc sulphate) .weremiixdsWith.leave-15.0: Zn'Clz ..solution .ofi50I-lBai1m i(equivalent' to. 128.5parts .by.weight.oiiZriClz)l andlthe.1nixture;placedin an 335-ordinarystrike' tank-eq1'iippd with. an agitator.

..Ammonium..su 1phidecalcium ,chlorideliquor .ob-

tained by leachingccalciumlblack .ash with'NI-IiClsolution,..containing;132 3. "(NH4) 58, per liter .and

' 240,g...CaClz, .per'liten was allowed to'iiow into the -zinc; liquornat. room temperature, -and while the reaction mixtureswassubjecte'd'to'agitation. ,The

addition 36f Lthe .ammonium "sulphide-calcium .hhloride liquor ;-was.stopped when the slurry showed .a .mdderatesulphide test-with nickelsul- I r'phate. lThisisnsu'ally atabout'a i fioffij. "The -raw ,pigmentwas subjected r to .Zfiltration .and washing ;.'for removal to!:wateresoluble salts, echifly. ammonium. chloride. Inofder to reduce the.quantity :of "wash "water, which. it .will' be necessary to. evaporatein. order? tozrecover; ammonium chloride fiior .use "in {preparing.IIIOIB (NI-I4) 'iS-CaClz .solution) ,Iiti is desirable to? have theoriginal -zinc orammonium"sulphide calcium chloride liquors-"6f highconcentration. Zincliqnor as high as' Baum'isconveniently -worked withwhen the ammoniumsulphide caicium chloride liquor 'contains.iabout 13%"ammonium sulphide, as above. s'After zwashing' freefrom'watersolublesaltsfthe'pigmentis'readyfordrying arid -calcinationin"the"customarymanner. In-case "theTH-oftheraw presscakeis' below'it-mi'ght be desirable to add a small *quan'tityof sodium hydroxide "orsodium *sulphide to the last wash water.

After calcination, themesultant Jni' ment on n analysis mas-ioundtto'contai-ni i'lluyo zinc sulphide .and2a5.1.62% ecalciumnsulphate. Thepigment was-sot excel-lent lcolor gradi-ng .1'5, and showed atintingstrength (if 164, which is very exceilent 'consideri-ng thefizin'osulphide content.

The polar =arid tinting strength figures given 'abovepas welhas thosehereinafter eset out, were =determined' imaccondance:with -themethodselescribedifiby Boogeziarrdrfiastiackr omipage. I1 (if-the April16, 1=924,': issue of'flPaint Dileand ChemicaI Review?colorsgmtdingzas'iewasi wander: the

arbitrary scale there employed .constitutes a .commercially saleablepigment. V

Example II To 1546 parts by volume of a purified zinc liq- 5 ';uor:.:containing -l81.-5 g. Zn, 225.2 g. S04 and w43.2 -g.' Clper literwere added, during efiicient agitation of the reaction mass, sufficientcalcium chloride solution free-of objectionable impurities, toprecipitate the S04 content of said zinc liquor. 10

To the reaction mass resulting from the above -operation- -wereadded,likewise with eflicient agi- "tation, 2160 parts by volume of anammonium sulphide solution containing 190 g. (NH4)2S/L.

The composite precipitate was filtered and '-;-washed toremove aconsiderable portion ofithe ..NH4C1 therefrom.

The recovery of NHiCl' was equivalent. to:90.2% of the (N114) 2S used.

The composite precipitate .waslthen :dried to lowmoisture content (5 to10%) and was'calmined in a non-oxidizing atmosphere.

When so calcined :at 775 C.:a compositezinc -sulphide calcium sulphatepigment 1 containing 47.84% ZnS and-51.16 CaSO4 resulted. :The' tint-.ingstrength of this product measured 160;:avery .satisfactory value forthis zincsulphidecontent.

Color graded 17, an exceptionally high ;=grade :product-with respect tocolor.

Example III To a purified zincsolution consisting of 1800 ,partsbyvolume of 45.B.-zinc sulphate and 265 parts by volume of 50 B. zincchloride-was added, during efiicient agitation,-2910 parts=by 35 volumeof a solution consisting of ammonium sul- ,phide andcalcium chloride.

The solutionconsisting of. ammonium sulphide -andcalcium chloridecontained g. .(-NH4) 2S and 240g. 'CaClz per liter and was obtainedbyleaching crudecalcium sulphidewith ammonium chloride solution resulting.from .the previous batch .of the operation. .The crude calcium-sulphidewas'obtained by reductionof calcium sulphate .with carbonaceous materialat elevated temperature.

.The precipitate consisting of.- zinc sulphide and calcium sulphatewas'filteredand washed with-a limited amount of Water to. recover.ammonium :chlorideforreuse'in the process. Ammoniumchloride equivalentto 89% of the .ammoni-um' 'fsulphide used in the process was recovered.

The precipitate was, then further washed, dried 'tof'a relatively lowmoisturecontent-and was calcined in'a non-oxidizing atmosphere.

When calcined at'750 C.' the resulting com- "positepigment containing48% ZnS and. 51 .57% "CaSOI. graded equal to 158 in tinting strength andequal to 16 in color.

-While preferablyl resort to a coeprecipitat-ion processfor theproduction .of my improved zinc sulphide-calcium sulphate pigment,wherein am- *mom'um sulphide and a cyclic operation is had which afiordsthe recovery of ammonium chloride for reuse in the process, saidcomposite pigment *mayalso'be obtained by employing other alkalisulphides, such as the metallic sulphides of, so- Ndium and potassium,to precipitatethezincsulphide. 'In the latter event, howeven'and inorder to obtain a commercially satisfactory composite JV pigment, I havefound it necessary to carry out the process by stepwise or successivereactions, and in such manner that the alkali sulphide is not' addeddirectly to the calcium chloride. 'For #example, I find it desirable tosuccessively precipitate the calcium sulphate and zincsulphidej i fi byreacting the zinc solution containing sulphate ions with the calciumchloride solution to precipitate calcium sulphate, and subsequentlyadding able because when solutions of alkali sulphide and solublecalcium salts are mixecL'a precipitate of Ca(OII)z results. Furthermore,by such successive precipitations, avoidance of the formation of calciumsulphide and soluble calcium sulphate is had. On the other hand, as hasbeen shown, when solutions of ammonium sulphide and calcium chloride aremixed, no precipitate results until the mixture is contacted with thezinc solution, and obviously in such pref rred process, the order ofaddition of reactants is unimportant.

In the instance of such successive precipitations and in order to obtaina commercially satisfactory pigment, I find it necessary to effect acareful control over and regulation of the reactants duringprecipitating reaction. Thus, in order to obtain a suitable calciumsulfate base of desired and necessary uniformity and particle sizeaverage, I find it preferable to maintain the zinc sulphate liquorduring reaction with CaClz at a "pl-I not to exceed substantially 6.5and preferably at a pl-I ranging from 5.5 to 6.2. Again, the liquorconcentrations must be maintained relatively constant, as must thetemperature, speed of agitation, and rate of reactant addition. In theinstance of sodium sulphide use, I find it desirable to maintain theliquor in heated state and between 70-90" C. Similarly, I prefer toemploy relatively concentrated solutions of sodium sulphide ranging from3G-50 Baurn, although the use of relatively dilute concentrationsbetween 4-10" Baume may be employed. In preferred practice, I find itdesirable to avoid variation in concentration beyond 1 Baum and 5 1. intemperature. As indicated, the limit of variation is merely preferable,and it must be understood that maintaining the concentration uniform atany other point, and within the range of solubility and of temperaturewithin, say, iii-90 0., is perinissible.

Example IV Zinc liquor containing zinc, sodium, sulphate and chlorideions was purified in the usual manner. This zinc solution represented aby-product obtained from the commercial recovery of copper from pyritessubjected to a chloridizing roast. Such a zinc solution maybe consideredas a mixture of zinc sulphate, zinc chloride and sodium chloride inaqueous solution.

To 49 parts by volume of the aforesaid zinc solution containing 115.06grams of zinc, 71.24 grams of sulphate and 174.16 grams of chlorine perliter, were added, while thoroughly agitating the same, 6 parts byvolume of a purified 54% calcium chloride solution. After precipitationof the available sulphate as calcium sulphate and Without removal ofsuch calcium sulphate from the reaction mass there were added, whilethoroughly agitating such reaction mass, 25 parts by volume of a 22 Baumsolution of sodium sulphide maintained at a temperature of 75 C'.

The addition of such quantity of sodium sulphide solution precipitatedall of the zinc as zinc sulphide and resulted, at the end ofprecipitation, in a slightly alkaline reaction mass.

The precipitate consisting essentially of zinc sulphide and calciumsulphate was then filtered and washed. The resulting washed precipitatewas found to contain. only traces of chloride.

After heat treating this pigment in an autoclave as described in U. S.Patent #1,977,582 to I-Ianahan, an excellent zinc sulphide-calciumsulphate pigment was developed having a tinting strength of 254 andcolor of 16. Such pigment was found by chemical analysis to contain70.64% of zinc sulphide, the remainder being calcium sulphate.

The above tinting strength grading shows that this pigment with onlyabout 70% of zinc sulphide content compares favorably with commercialzinc sulphide pigments consisting almost entirely of zinc sulphide.

As will be noted from the analyses of my pigment products, the calciumcontent of such products is substantially such as may be calculated willresult when employing zinc solutions of the zinc and sulphate contentspecified. The only slightly lower than theoretical calcium sulphatecontent is explained by the normal solubility of calcium sulphate inwater. It should also be noted that my process does not necessitateresort to use or" addition agents such as alkali sulphates, for thepurpose of restraining the solubility of calcium sulphate in thereaction liquors. This is a distinct advantage, because the introductionof sodium salts into the zinc sulphide-containing pigment, todeleteriously act as fritting agents during calcination, is thusavoided.

I claim as my invention:

1. In a process for producing a composite calcium sulphate-zinc sulphidepigment, the step of reacting a zinc sulphate solution with an aqueoussolution containing ammonium sulphide and calcium chloride.

2. A process for co-precipitating insoluble calcium sulphate and zincsulphide to obtain a composite pigment, comprising reacting a solutionof zinc sulphate with an aqueous calcium salt solution containingammonium sulphide.

3. A process for producing a composite calcium sulphate-zinc sulphidepigment, comprising reacting solutions of zinc sulphate with aqueouscalcium salt solutions containing ammonium sulphide, while maintainingthe reaction mixture at a pH value not exceeding 6.5.

l. A process for producing a composite calcium sulphate-zinc sulphidepigment, comprising reacting a solution of zinc sulphate with aqueouscalciurn salt solutions containing ammonium sulphide, While maintainingthe reaction mixture at a pH value ranging from 5.5 to 6.2.

ROY WILLIAM SULLIVAN.

